Recently, for improving vehicle fuel efficiency and reducing CO2 emission, the development of technology has been actively made to reduce an engine displacement (engine downsizing) or increase a gear ratio (high geared) and ensure power performance by a supercharger such as a turbocharger.
As a turbocharger system used at this time, there has been proposed a two-stage turbo that uses two turbochargers having different capacities and switches a turbocharger to be used in an operating zone. As the two-stage turbo, there are a parallel two-stage turbo in which two turbochargers are connected in parallel, and a series two-stage turbo in which two turbochargers are connected in series (see, for example, Patent Document 1).
In the series two-stage turbo, two-stage supercharging using both of a high-pressure stage turbocharger and a low-pressure stage turbocharger is enabled. Therefore, as compared with the parallel two-stage turbo, the series two-stage turbo is characterized in that it is possible to obtain a high boost pressure in a middle-low speed range.